Aqueous developable deep UV negative resist containing benzannelated acetic acid and novolak resin

ABSTRACT

A radiation-sensitive mixture useful as a negative-working photoresist composition comprising: 
     (a) at least one novolak resin; and 
     (b) a photoactive benzannelated acetic acid selected from formula (I): ##STR1##  wherein X is either an oxygen, sulfur or ##STR2##

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to radiation-sensitive mixtures useful asnegative-working resist compositions containing at least one novolakresin and selected benzannelated acetic acids as the photoactivecompound. Furthermore, the present invention also relates to substratescoated with these radiation-sensitive mixtures as well as the process ofimaging and developing these radiation-sensitive mixtures on thesesubstrates.

2. Description of Related Art

Photoresist compositions are used in microlithographic processes formaking miniaturized electronic components such as in the fabrication ofintegrated circuits and printed wiring board circuitry. Generally, inthese processes, a thin coating or film of a photoresist composition isfirst applied to a substrate material, such as silicon wafers used formaking integrated circuits or aluminum or copper plates of printedwiring boards. The coated substrate is then baked to fix the coatingonto the substrate. The baked coated surface of the substrate is nextsubjected to an image-wise exposure of radiation. This radiationexposure causes a chemical transformation in the exposed areas of thecoated surface. Ultraviolet (UV) light, electron beam and X-ray radiantenergy are radiation types commonly used today in microlithographicprocesses. After this image-wise exposure, the coated substrate istreated with a developer solution to dissolve and remove either theradiation-exposed or the unexposed areas of the coated surface of thesubstrate.

There are two types of photoresist compositions--negative-working andpositive-working. When negative-working photoresist compositions areexposed image-wise to radiation, the areas of the resist compositionexposed to the radiation become more insoluble to a developer solution(e.g. a cross-linking reaction occurs) while the unexposed areas of thephotoresist coating remain relatively soluble to a developing solution.Thus, treatment of an exposed negative-working resist with a developercauses removal of the non-exposed areas of the resist coating and thecreation of a negative image in the photoresist coating. On the otherhand, when positive-working photoresist compositions are exposedimage-wise to radiation, those areas of the resist composition exposedto the radiation become more soluble to the developer solution (e.g. adecomposition reaction occurs) while those areas not exposed remainrelatively insoluble to the developer solution. Thus, treatment of anexposed positive-working resist with the developer causes removal of theexposed areas of the resist coating and the creation of a positive imagein the photoresist coating.

After this development operation, the now partially unprotectedsubstrate may be treated with a substrate-etchant solution or plasma gasmixture and the like. The etchant solution or plasma gas mixture etchesthe portion of the substrate where the photoresist coating was removedduring development. The areas of the substrate where a positivephotoresist coating still remains are protected and, thus, an etchedpattern is created in the substrate material which corresponds to thephotomask used for the image-wise exposure of the radiation. Later, theremaining areas of the positive photoresist coating may be removedduring a stripping operation, leaving a clean etched substrate surface.

Positive-working photoresist compositions are currently favored overnegative-working resists because the former generally have betterresolution capabilities and pattern transfer characteristics.

Photoresist resolution is defined as the smallest feature which theresist composition can transfer from the photomask to the substrate witha high degree of image edge acuity after exposure and development. Inmany manufacturing applications today, resist resolution on the order ofone micron or less is necessary.

In addition, it is generally desirable that the developed photoresistwall profiles be near vertical relative to the substrate. Suchdemarcations between developed and undeveloped areas of the resistcoating translate into accurate pattern transfer of the mask image ontothe substrate.

Still further, many current negative photoresist formulations also swellwhen subjected to development steps, thereby causing image distortion.And, negative photoresists generally require an organic developersolution. The employment of such organic materials creates specialhandling and disposal problems for the photoresist fabricator.

On the other hand, positive photoresist formulations are not favored forall commercial applications. For example, positive photoresists such asthose based on novolak resins and orthonaphthoquinone diazidephotosensitizers have certain processing limitations when their imagingis carried out in the deep ultraviolet region of the light spectrum. Inthis class of positive resists, both ingredients absorb light from thedeep ultraviolet region and, thus, the photoresist requires increasedinput of radiation to compensate for the unwanted light absorptions.

Accordingly, there is a need for a better negative-working photoresistformulation which overcomes the deficiencies of current negative-workingphotoresists, especially in the area of the deep UV light region wherepositive-working resists have limitations as to commercialization. Thepresent invention is believed to be an answer to that need.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a radiation-sensitivemixture useful as a negative-working photoresist composition comprisingthe admixture of:

(a) at least one novolak-type resin; and

(b) a photoactive benzannelated acetic acid selected from the compoundsof formula (I): ##STR3## wherein X is either an oxygen, sulfur or##STR4##

Moreover, the present invention also encompasses the process of coatingsubstrates with these radiation-sensitive mixtures and then imaging anddeveloping these coated substrates.

Still further, the present invention also encompasses said coatedsubstrates (both before and after imaging) as novel articles ofmanufacture.

DETAILED DESCRIPTION

The preferred photoactive (also called "sensitizer") benzannelatedacetic acid is xanthene-9-carboxylic acid. This compound is also knownas 9H-xanthene-9-carboxylic acid, xanthenecarboxylic acid, or xanthanoicacid and its Chemical Abstracts Registry Number is 82-07-5. Itsstructure is shown by Formula II: ##STR5##

Its sulfur analog is known as thioxanthene-9-carboxylic acid or9H-thioxanthene-9-carboxylic acid and its Chemical Abstract RegistryNumber is 17394-14-8.

The photoactive benzannelated acetic acid is then combined with anovolak resin or resins to make radiation-sensitive mixtures useful asnegative-working photoresist compositions. The term "novolak-type resin"is used herein to mean any novolak resin which is conventionally used inphotoresist compositions. Suitable novolak resins includephenol-formaldehyde novolak resins and cresol- formaldehyde novclakresins, preferably having a molecular weight of about 500 to about30,000, more preferably from about 1,000 to about 20,000. These novolakresins are preferably prepared by the condensation reaction of phenol orcresol with formaldehyde and are characterized by being light-stable,water-insoluble, alkali-soluble and film-forming. The preparation ofexamples of such suitable resins is disclosed in U.S. Pat. Nos.4,377,631; 4,529,682; and 4,587,196, all of which issued to MedhatToukhy and are incorporated herein by reference in their entireties.

The proportion of the above sensitizer compound in theradiation-sensitive mixture may preferably range from about 1 to about30 percent, more preferably from about 5 to about 25 percent by weightof the non-volatile (e.g. non-solvent) content of theradiation-sensitive mixture. The proportion of novolak resin in theradiation-sensitive mixture may preferably range from about 70 to about99 percent, more preferably, from about 75 to 95 percent of thenon-volatile (e.g. excluding solvents) content of theradiation-sensitive mixture.

These radiation-sensitive mixtures may also contain conventionalphotoresist composition ingredients such as solvents, actinic andcontrast dyes, anti-striation agents, plasticizers, speed enhancers, andthe like. These additional ingredients may be added to the novolak resinand sensitizer solution before the solution is coated onto thesubstrate.

The resins and sensitizers may be dissolved in a solvent or solvents tofacilitate their application to the substrate. Examples of suitablesolvents include ethyl cellosolve acetate, n-butyl acetate, xylene,ethyl lactate, propylene glycol alkyl ether acetates, or mixturesthereof and the like. The preferred amount of solvent may be from about50% to about 500% by weight, more preferably, from about 100% to about400%, based on combined resin and sensitizer weight.

Actinic dyes help provide increased resolution by inhibiting backscattering of light off the substrate. This back scattering causes theundesirable effect of optical notching, especially where the substrateis highly reflective or has topography. Examples of actinic dyes includethose that absorb light energy at approximately 400-460 nm [e.g. FatBrown B (C. I. No. 12010); Fat Brown RR (C. I. No. 11285);2-hydroxy-1,4-naphthoquinone (C. I. No. 75480) and Quinoline Yellow A(C. I. No. 47000)] and those that absorb light energy at approximately300-340 nm [e.g. 2,5-diphenyloxazole (PPO-Chem. Abs. Reg. No. 92-71-7)and 2-(4-biphenyl)-6-phenyl-benzoxazole (PBBO-Chem. Abs. Reg. No.17064-47-0)]. The amount of actinic dyes may be up to ten percent weightlevels, based on the combined weight of resin and sensitizer.

Contrast dyes enhance the visibility of the developed images andfacilitate pattern alignment during manufacturing. Examples of contrastdye additives that may be used together with the radiation-sensitivemixtures of the present invention include Solvent Red 24 (C. I. No.26105), Basic Fuchsin (C. I. 42514), Oil Blue N (C. I. No. 61555) andCalco Red A (C. I. No. 26125) up to ten percent weight levels, based onthe combined weight of resin and sensitizer.

Anti-striation agents level out the photoresist coating or film to auniform thickness. This is important to ensure uniform radiationexposure over the film surface. Anti-striation agents may be used up tofive percent weight levels, based on the combined weight of resin andsensitizer. One suitable class of anti-striation agents is non-ionicsilicon-modified polymers. Non-ionic surfactants may also be used forthis purpose, including, for example, nonylphenoxy poly(ethyleneoxy)ethanol; octylphenoxy (ethyleneoxy) ethanol; and dinonyl phenoxypoly(ethyleneoxy) ethanol.

Plasticizers improve the coating and adhesion properties of thephotoresist composition and better allow for the application of a thincoating or film of photoresist which is smooth and of uniform thicknessonto the substrate. Plasticizers which may be used include, for example,phosphoric acid tri-(β-chloroethyl)-ester; stearic acid; dicamphor;polypropylene; acetal resins; phenoxy resins; and alkyl resins up to tenpercent weight levels, based on the combined weight of resin andsensitizer.

Speed enhancers tend to increase the solubility of the photoresistcoating in both the exposed and unexposed areas, and thus, they are usedin applications where speed of development is the overridingconsideration even though some degree of contrast may be sacrificed,i.e. in negative resists while the unexposed areas of the photoresistcoating will be dissolved more quickly by the developer, the speedenhancers will also cause a larger loss of photoresist coating from theexposed areas. Speed enhancers that may be used include, for example,picric acid, nicotinic acid or nitrocinnamic acid at weight levels of upto 20 percent, based on the combined weight of resin and sensitizer.

The prepared radiation-sensitive resist mixture, can be applied to asubstrate by any conventional method used in the photoresist art,including dipping, spraying, whirling and spin coating. When spincoating, for example, the resist mixture can be adjusted as to thepercentage of solids content in order to provide a coating of thedesired thickness given the type of spinning equipment and spin speedutilized and the amount of time allowed for the spinning process.Suitable substrates include silicon, aluminum or polymeric resins,silicon dioxide, doped silicon dioxide, silicon resins, galliumarsenide, silicon nitride, tantalum, copper, polysilicon, ceramics andaluminum/copper mixtures.

The photoresist coatings produced by the above described procedure areparticularly suitable for application to thermally grown silicon/silicondioxide-coated wafers such as are utilized in the production ofmicroprocessors and other miniaturized integrated circuit components. Analuminum/aluminum oxide wafer can be used as well. The substrate mayalso comprise various polymeric resins especially transparent polymerssuch as polyesters and polyolefins.

After the resist solution is coated onto the substrate, the coatedsubstrate is baked at approximately 70° to 115° C. until substantiallyall the solvent has evaporated and only a uniform radiation-sensitivecoating remains on the substrate.

The coated substrate can then be exposed to radiation, especiallyultraviolet radiation, in any desired exposure pattern, produced by useof suitable masks, negatives, stencils, templates, and the like. Anyconventional imaging process or apparatus currently used in processingphotoresist-coated substrates may be employed with the presentinvention.

The exposed resist-coated substrates are next developed in alkalineinorganic or organic developing solution. Immersion development ispreferred. This solution is preferably agitated, for example, bynitrogen gas agitation during immersion. Examples of alkaline inorganicdevelopers include aqueous solutions of tetramethylammonium hydroxide,sodium hydroxide, potassium hydroxide, chlorine, sodium phosphates,sodium carbonate, sodium metasilicate, and the like. Examples of organicdevelopers include isopropanol alone or mixed with methyl isobutylketoneor mixtures of methyl ethyl ketone, ethanol and isopropanol and thelike. The preferred developers for this invention are aqueous solutionsof tetramethylammonium hydroxide.

Alternative development techniques such as spray development or puddledevelopment, or combinations thereof, may also be used.

The substrates are allowed to remain in the developer until all of theresist coating has dissolved from the unexposed areas. Normally,development times from about 30 seconds to about 3 minutes are employed.

After selective dissolution of the coated wafers in the developingsolution, they are preferably subjected to a deionized water rinse tofully remove any remaining undesired portions of the coating and to stopfurther development. This rinsing operation (which is part of thedevelopment process) may be followed by blow drying with filtered air toremove excess water. A post-development heat treatment or bake may thenbe employed to increase the coating's adhesion and chemical resistanceto etching solutions and other substances. The post-development heattreatment can comprise the oven baking of the coating and substratebelow the coating's softening point.

In industrial applications, particularly in the manufacture ofmicrocircuitry units on silicon/silicon dioxide-type substrates, thedeveloped substrates may then be treated with a buffered, hydrofluoricacid etching solution or plasma gas etch. The resist compositions of thepresent invention are believed to be resistant to a wide variety of acidetching solutions or plasma gases and provide effective protection forthe resist-coated areas of the substrate.

Later, the remaining areas of the photoresist coating may be removedfrom the etched substrate surface by conventional photoresist strippingoperations.

The present invention is further described in detail by means of thefollowing Examples. All parts and percentages are by weight unlessexplicitly stated otherwise.

EXAMPLE 1 PREPARATION OF PHOTORESIST FORMULATION

3.52 grams of Xanthene-9-carboxylic acid were mixed with a solutioncomprising 15.00 grams mixed 45% m- and 55% p-cresol formaldehydenovolac resin (weight average M.W. of about 4929) and 51.84 grams ofethyl lactate. The bottle was then rolled for 12 hours at roomtemperature until all the solids were dissolved. The resulting resistsolution was then filtered through a 0.2 micron pore size filter using aMillipore microfiltration system (a 100 ml. barrel and a 47 mm. diskwere used). The filtration was conducted in a nitrogen atmosphere undera gauge pressure of 10 pounds per square inch.

EXAMPLE 2 COATING OF PHOTORESIST COMPOSITION ONTO A SILICON WAFER

Approximately three mls. of the filtered resist composition in Example 1was spin-coated with a Model 5110-C single head spinner manufactured bySolitec, Inc. (Santa Clara, Calif.) onto a thermally grownsilicon/silicon dioxide-coated wafer of four inches in diameter andhaving 5400 Angstroms of silicon dioxide on its upper surface which wasprimed with 20% by volume hexamethyldisilazane (HMDS)/80% by volumexylene solution. The resist was applied to a static wafer. Then, thewafer was rotated to an initial spinning velocity of 500 revolutions perminute for 3 seconds, followed by acceleration at 2,000 revolutions persecond to a final spinning velocity of 1,500 revolutions per minute for30 seconds. This spinning operation evenly spread the photoresist overthe upper surface of the wafer to produce an even thin film. The coatedwafer was then subsequently baked at 100° C. on a vacuum applied hotplate for 60 seconds. The photoresist film thickness was then measuredto be approximately one micron with a Dektak IIa profilometer unitmanufactured by Sloan Technology (Santa Barbara, Calif.).

EXAMPLE 3 DEEP-UV IMAGE-WISE EXPOSURE OF 100° C. BAKED COATED WAFER

The wafer baked at 100° C. in Example 2 was image-wise exposed todeep-UV light wavelengths between 220-250 nm using a Canon ModelPLA-501F aligner (Lake Success, N.Y.) equipped with a Xenon-mercury UVlamp and CM250 cold mirror for a twenty five second exposure time in thehard contact mode . The deep-UV wavelengths were passed through a quartzSeries 1 multidensity resolution target from Detric Optics, Inc.(Hudson, Mass.). Eight-tenths of a micron features were confirmed on thetarget using scanning electron microscopy. The intensity at the waferplane was measured to be 8.62 mW/cm² with a Mimir Instruments Inc.(Santa Clara, Calif.) Model 100 Powermeter equipped with a detector formeasurement at 254 nm.

EXAMPLE 4 DEVELOPMENT OF EXPOSED RESIST COATED WAFER

The resist coated wafer exposed according to Example III was then heldwith Teflon tweezers and immersed in a 500 milliliter polypropylenecontainer containing 25% by volume WAYCOAT Positive MIF Developersolution (Olin Hunt Specialty Products, Inc., West Paterson, N. J.) inwater. This WAYCOAT solution is an aqueous solution oftetramethylammonium hydroxide. The wafer was allowed to remain immersedin the developer solution while the container was moved in a circularmotion for fifty five seconds. Then the wafer was rinsed in deionizedwater for one minute and dried in a stream of filtered nitrogen. Theunexposed areas of the photoresist film were developed thereforeproducing a negative image.

The developed and exposed wafer was then examined to determine thephotospeed of the photoresist film and small feature sizes.

Photospeed of the resist was determined by looking at each of thedeveloped areas of the resist coating corresponding to different percenttransmittance windows of the SERIES I target. Photospeed of this resistwas calculated by multiplying the exposure energy at the wafer plane(8.62 milliwatts per square centimeter), the lowest percenttransmittance window of the target at which the resist fully coated, andthe full transmittance time (twenty five seconds) and then dividing by100.

Examination of fine features was done using a Nikon optical microscopewith one thousand times magnification.

The first panel to be fully coated was the 25.1% window corresponding toa photospeed of 54.1 millijoules per centimeter squared at 254 nm. Theoptical microscope revealed eight-tenths micron lines.

The measured photospeed and fine feature size indicates this photoresistformulation which was baked at 100° C. and imaged in the Deep-UV rangeshould provide excellent resolution and, thus, appears suitable forcommercial applications where these baking temperatures are employed.

WHAT IS CLAIMED IS:
 1. A radiation-sensitive mixture useful as anegative-working photoresist composition comprising the admixture:(a) atleast one novolak resin; and (b) a photoactive benzannelated acetic acidselected from formula (I): ##STR6## wherein X is either an oxygen,sulfur or ##STR7##
 2. The radiation-sensitive mixture of claim 1 whereinsaid novolak resin is selected from the group consisting ofphenol-formaldehyde novolak resins and cresol-formaldehyde resins. 3.The radiation-sensitive mixture of claim 1 wherein (a) is from about 70to about 99 percent and (b) is from about 30 to about 1 percent of thenon-volatile content of said mixture.
 4. The radiation-sensitive mixtureof claim 1 wherein said benzannelated acetic acid isxanthene-9-carboxylic acid.
 5. The radiation-sensitive mixture of claim1 further comprising one or more additives selected from the groupconsisting of solvents, actinic and contrast dyes, anti-striationagents, plasticizers and speed enhancers.